Your calculations on when she should have ovulated were WAY OFF. Sorry for yelling, but judging from Apollo's post, you and your girlfriend are young and obviously don't know much about a woman's cycle or safe sex. In general and on average, for a woman with a 28-day cycle, you girlfriend's ovulation would have taken place on the 14th.
As for the 'safe sex' part. Calculating when a woman is ovulating and trying to avoid that time is called the Rhythm Method and is a completely unreliable form of birth control. It is probably thee worst way of trying to avoid pregnancy. And your other method of birth control, the withdrawal method, isn't much better. If your girlfriend isn't pregnant now, if you keep that up, she will be in no time.
However, now that I'm done ripping apart your choice of birth control, I'll go on a bit more about her "symptoms". If you had unprotected sex with her on either the 12th, 13th or 14th, and her symptoms started on the 13th, that's pretty quick (especially if you had sex with her on the 14th). But it is possible if intercourse was before then. As for the nipples getting darker, that doesn't happen right away and it's sometimes a very subtle change.
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where do i start? 
Linear Mode
